Nickel metal hydride (NiMH) batteries have long cycle life and have rapid charge and discharge capabilities. During charge and discharge the electrodes interact with each other through the alkaline electrolyte as hydrogen is transported in the form of water molecules between the electrodes. During discharge hydrogen is released from the negative electrode and is allowed to migrate to the positive electrode (nickel electrode) where it intercalates. This binding results in energy being released. During charging the hydrogen migration is reversed, see FIG. 1.
Especially NiMH batteries are designed to be nickel electrode limited with a starved electrolyte. This is done in order to be able to avoid overcharge and overdischarge states of the battery cells by controlling the cell chemistry and state-of-charge via the gas phase.
When the cell is charged, hydrogen is transported from the nickel hydroxide to the metal hydride by water molecules in the aqueous alkaline electrolyte. During discharge hydrogen is transported back to the nickel hydroxide electrode, again in the form of water molecules.
If the cell is charged beyond the capacity of the nickel electrode, hydrogen will still be transported and intercalated into the metal hydride electrode by water molecules, but in this case hydrogen will be taken from the aqueous electrolyte resulting in a production of oxygen gas. The overcharging reaction is thus denoted: 40H−=2H2O+O2+4e− (E0=+0.401V). A cell with a starved electrolyte means in contrast to a flooded cell, that the amount of electrolyte is so limited that open spaces and channels exists between the electrodes through the separator. These open channels can now transport the oxygen to the metal hydride electrodes, where it can be recombined to form water. This recombination reaction is denoted: 2MH+O2=2H2O+2M. The metal hydride electrode has thus a certain overcharge capacity reserve in relation to the nickel electrode.
If the cell on the other hand is overdischarged, hydrogen will be transported to the nickel electrode. But as the capacity of the nickel electrode is below that of the metal hydride electrode, hydrogen will be released as hydrogen gas molecules instead of being intercalated into the nickel hydroxide. These hydrogen gas molecules can also migrate through the open channels to the metal hydride electrode and be recombined into water. A certain overdischarge capacity of the metal hydride electrode is usually created by adding cobalt to the nickel electrode, which results in a controlled pre-charging of the metal hydride electrode during the formation of the battery cells.
A proper balance of the nickel electrode capacity with respect to the metal hydride electrode capacity with suitable amounts of both overcharge- and overdischarge reserves are essential for a well-functioning battery, enabling it to reach a stable long time charge/discharge performance, FIG. 2.
This essential balancing of the two electrodes' capacity with respect to each other is unfortunately impaired by several mechanisms as the battery cells age.